The objective of these studies is to describe the molecular events and controlling mechanisms during the differentiation of lens fibers in chick embryos. This system has been chosen and developed as a model for cellular differentiation because it contains a single cell type, undergoes simple, defined morphological changes, synthesizes large amounts of specific proteins and differentiates in vitro. Within the past year, we have demonstrated a unique pattern of histones exists in differentiating lens cell nuclei, which consists of a large amount of characteristic F1 histone. We have described a changing pattern of protein synthesis during lens fiber cell differentiation in vitro, and established that delta-crystallin synthesis is differentially stimulated. We determined the size of delta-crystallin mRNA, synthesized a delta-crystallin cDNA and showed that the accumulation of delta-crystallin mRNA in the cytoplasm quantitatively accounts for the differential stimulation of delta-crystallin synthesis in tissue culture. Finally, we have demonstrated that newly synthesized delta- crystallin is posttranslationally modified in the embryonic chick lens.